Recent years, in the field of micro electrical mechanical system (MEMS), the manufacture of devices containing functional materials such as electronic ceramics, which express predetermined functions by being applied with voltages, like dielectric materials, piezoelectric materials, magnetic materials, pyroelectric materials, and semiconductor materials by using film formation technologies has been actively studied.
For example, in order to enable high-definition and high-quality printing in an inkjet printer, it is necessary to miniaturize and highly integrate ink nozzles of an inkjet head. Accordingly, it is also necessary to similarly miniaturize and highly integrate piezoelectric actuators for driving the respective ink nozzles. In this case, a film formation technology that enables formation of a thinner layer than a bulk material and formation of fine patterns is advantageous.
Recently, as one of the film formation technologies, the aerosol deposition method (hereinafter, referred to as “AD method”) known as a technology for forming a film of ceramics, metals and so on has received attention. The AD method is a film forming method of depositing a raw material on a substrate by dispersing powder of the raw material (raw material powder) in a gas (aerosolizing) and injecting it toward the substrate from a nozzle. Here, the aerosol refers to solid or liquid microparticles floating in a gas. The AD method is also referred to as “injection deposition method” or “gas deposition method”.
As a related technology, Japanese Patent Application Publication JP-P2002-235181A (page 2) discloses a method of fabricating a composite structure including, after the step of applying internal strain to brittle material microparticles, the steps of allowing the brittle material microparticles applied with the internal strain to collide with a base material surface at a high speed for deforming or crushing the brittle material microparticles by the impact of the collision, rebinding the microparticles via active newly-formed surfaces formed by the deformation or crushing and thereby forming an anchor part made of a polycrystalline brittle material, a part of which cuts into the base material surface, at the boundary part between the brittle material and the base material, and subsequently forming a structure made of a polycrystalline brittle material on the anchor part.
As disclosed in JP-P2002-235181A, according to the AD method, the substrate and the structure formed thereon are brought into strong and close contact due to the presence of the anchor part. Further, the film formation mechanism of binding the microparticles on the active newly formed surfaces formed at the time of collision is called mechanochemical reaction. Since a dense and strong film can be formed according to the AD method, it is expected that the performance of devices applied with various kinds of functional films is improved.
Further, Japanese Patent Application Publication JP-P2005-36255A (pages 1, 6, 8 and 11) discloses a method of fabricating a composite structure including the steps of performing energy application such as plasma application or microwave application on microparticles of a brittle material in a reduced-pressure atmosphere, and then, injecting an aerosol formed by dispersing the microparticles of the brittle material applied with energy in a gas from a nozzle toward a base material so that the aerosol collides with a surface of the substrate to crush and deform the microparticles and bond the microparticles to the substrate due to the impact of the collision, and thereby forming a structure made of the constituent material of the microparticles on the base material.
In JP-P2005-36255A (page 11), in order to strongly bond the microparticles colliding with the substrate or the like, the microparticle surfaces are activated by applying energy of plasma or the like to the microparticles before aerosolization to remove impurity containing physisorbed water or chemisorbed water (water molecules hydrogen-bonding to hydroxyl groups and so on in the microparticle surfaces) and organic materials adhering to the surfaces of the microparticles. Further, as a result, mixture of impurities into the formed structure can be also prevented. Furthermore, JP-P2005-36255A (pages 6 and 8) also discloses that, in order to improve the speed of structure formation, a chemisorption layer is formed by using a steam generator on the surfaces of the microparticles after the impurities are once removed.
By the way, when a piezoelectric material such as PZT (lead zirconium titanate) is fabricated by using the AD method, it is necessary to heat-treat (post-anneal) the piezoelectric material after film formation because the piezoelectric material does not exhibit a sufficient electric property as it is. The reason is that the piezoelectric material exhibits a better piezoelectric property with a larger crystal particle diameter, and the crystal grain growth is promoted by the heat treatment. The relationship between the crystal particle diameter and the piezoelectric performance is described in Kikuchi et al., “Photostrictive Characteristics of Fine-Grained PLZT Ceramics Derived from Mechanically Alloyed Powder”, Journal of the Ceramic Society of Japan, Vol. 112, No. 10 (2004), pp. 572-576.
However, when a film formed by using the AD method, that is, an AD film is heat-treated at a predetermined temperature (typically, a higher temperature than the film formation temperature), sometimes the film is separated from the substrate in spite of the presence of the anchor part. Alternatively, sometimes a phenomenon called “hillock” that the film is partly expands occurs at the time of heat treatment.
Although the post-anneal is essential for improving the electric property of the piezoelectric material, when such a phenomenon occurs, it becomes impossible to use the formed film as the piezoelectric material. Accordingly, it is conventionally impossible to heat-treat the AD film at a high temperature (e.g., 1000° C.), nor make a particle diameter of the PZT larger than 500 nm, for example.